SEOUL (Yonhap) -- The United States was thrown yet another variable in its dealings with North Korea on Dec. 29 when the communist state said it has detained an American citizen for illegally entering the country across its border with China.

North Korea's official news agency said that it was investigating the U.S. citizen, believed to be Christian human rights activists Robert Park, Pyongyang's first response since he entered the country on Christmas Day. The incident comes amid efforts by the U.S. to bring North Korea back to the six-party talks on ending its nuclear weapons programs.

Park, a Korean-American missionary from Tucson, Arizona, entered North Korea on Dec. 25 singing a hymn and carrying a letter urging the North Korean leader, Kim Jong-il, to release all political prisoners from the country's concentration camps, according to human rights activists in Seoul. They cited two activists who had witnessed his crossing from the Chinese side of the river border.

Several North Korean concentration camps are believed to accommodate tens of thousands of political prisoners.

But the brief dispatch from the North's official Korean Central News Agency (KCNA) did not identify the person, who it said was under interrogation by the authorities. "A U.S. citizen has been detained by authorities after illegally entering the DPRK through the DPRK-China border on Dec. 24," the KCNA said, adding that an investigation is under way.

According to the activists, who claim to be members of a coalition of more than 100 groups focused on assisting North Korean defectors and addressing the country's notorious human rights conditions, Robert Park crossed the frozen Tumen River at around 5 p.m. on Dec. 25.

The 28-year-old missionary said in Seoul just days before entering the North that he did not care if he was killed in the North as long as it highlighted the plight of the North Koreans, according to reports.

Park was carrying a statement calling for Kim to step down immediately for starving his people and executing political prisoners, they said. North Korean border guards apparently detained Park soon after he entered the country, the activists said.

No accurate data on the North's human rights situation is available as the communist nation strictly controls traffic across its border. But the U.N. and global human rights groups say that citizens there have no freedom of speech and dissidents suffer torture and execution without trial.

A U.S. State Department spokesman, Ian Kelly, said in Washington that the Swedish Embassy in Pyongyang has offered to try to get information about Park for the U.S., which does not have diplomatic ties with North Korea.

"The DPRK government has confirmed it is holding a U.S. citizen pending an investigation," State Department spokesman Ian Kelly said in statement on Dec. 29. "We will continue to work through the Swedish Embassy, our protecting power in Pyongyang, to seek consular access to this American citizen."

Earlier this year, the Swedish Embassy helped secure the release of two American journalists detained after illegally entering the North accidentally across the Chinese border while reporting on North Korean defectors.

Their release was also facilitated by former U.S. President Bill Clinton, who visited North Korea and met with its leader, Kim Jong-il, in August. That led to a visit by Stephen Bosworth, special representative for North Korea policy, earlier this month in the first high-level contact since President Obama's inauguration in January.

Radio Free Asia reported Dec. 30 that the U.S. is discussing Park's release through the "New York channel," where Pyongyang has its representative mission at the United Nations.

The incident comes amid efforts by the U.S. to bring a reluctant North Korea back to the six-party talks on ending its nuclear weapons programs. The talks involve the two Koreas, the U.S., China, Japan and Russia.

The U.S. State Department said Dec. 30 that the detention of the American missionary in North Korea is a consular issue that is unrelated to security matters, apparently hoping the incident will not affect its efforts to bring the North back to the six-party talks. "We consider this to be a consular issue unrelated to any security or political issues," said Darby Holladay, a department spokesman.

North Korea has boycotted the multilateral nuclear talks due to U.N. sanctions over its missile and nuclear tests, demanding the nuclear issue be resolved through bilateral talks with the U.S.

Washington believes the six-party talks are the best way to address the North Korean nuclear issue, but has expressed its willingness to have another high-level, face-to-face meeting with Pyongyang to coax the North back to the negotiations.

In the face of skepticism that North Korea will continue dragging its feet under the six-party talks' cover, U.S. officials have said they will not reward the North just for returning to the table, and reiterated that sanctions will continue until the North takes substantial steps toward denuclearization.

Bosworth failed to obtain a commitment from the North to return to the six-party talks, but said that officials in Pyongyang "indicated they would like to resume the six-party process," and "agreed on the essential nature of the joint statement of 2005."

The 2005 deal calls for North Korea's nuclear dismantlement in return for massive economic aid, diplomatic recognition by Washington and Tokyo and establishment of a peace regime to replace the armistice that ended the 1950-53 Korean War.

U.S. officials have said they are ready to have another high-level, face-to-face meeting with the North to pave the way for reopening the multilateral nuclear talks, but skeptics predict North Korea will continue dragging its feet even if it returns.